1. Field of the Invention
This invention relates to variable displacement pumps and more particularly, to such pumps which are of a rotary nature. The invention has been conceived in connection with an emergency hydraulic power supply for aircraft and will, in the main, be discussed in relation thereto but the invention is not restricted to such an application.
2. Description of the Prior Art
In aircraft, hydraulic power is used to move the control ailerons, etc., and in most types of aircraft, it is a requirement that an emergency source of hydraulic power be provided which can be used in the event of a failure of the main hydraulic power system. To this end, it is known to employ a prime mover, such as a ram air turbine, for a variable displacement hydraulic pump, the prime mover powering the hydraulic pump in order to provide emergency hydraulic power for the control ailerons, etc. However, with a prime mover which is dependent upon the airspeed of the aircraft, then as the aircraft loses airspeed in an emergency situation, the prime mover loses power and the alternative hydraulic power supply can be lost at a relatively early stage because with a variable displacement pump, a pressure compensator is normally provided which ensures that the pump delivers hydraulic fluid at the flow rate demanded by the system and at a predetermined pressure. Accordingly, if the outlet pressure of the pump falls due to decreasing airspeed, then the pump will automatically try to increase that pressure by increasing the stroke of the pump, resulting in increased pump demanded power leading to stalling of the prime mover. Clearly, this is not acceptable with an emergency hydraulic supply and it is the principal object of the present invention to obviate this problem.